Conventional electronic tuning devices comprise a pair of identical resonant circuits disposed on a dielectric substrate, each resonant circuit including a conductive elongated film serving as a transmission line and a voltage-controlled capacitive element. The transmission lines of the resonant circuits are arranged in parallel to each other. Microwave energy is coupled with one of the resonant circuit and the tuned signal is then coupled with the other resonant circuit.
It is necessary however that the image interference ratio of the tuning device must be reduced to an acceptable level. An electronic tuning device as disclosed in Japanese Patent Application 50-151489 (laid open to public inspection on July 23, 1977 as Publication No. 52-75153) includes an image frequency trapping circuit which partially extends parallel with a pair of resonating transmission lines and partially extends at right angles crossing one of the transmission lines.
However, the cross-coupled transmission line undergoes a change in its characteristic impedance with a resultant discrepancy between the individual resonant frequencies of the two transmission lines, causing a tracking failure and waveform distortion.